Men of Gondor: The Men who dwelt in Gondor and defended the western World from Sauron, in part descended from Elendil and his followers, who founded Gondor after the Downfall of Númenor.
Gondor: The second of the two great nations of Men founded in Middle-earth by Elendil after the Downfall of Númenor, the other being Arnor in the north. Gondor was at first ruled jointly by Elendil's sons Isildur and Anárion. Both Elendil and Anárion were lost in the Siege of Barad-dûr, and his brother Isildur soon afterwards in the Disaster of the Gladden Fields, and so the Kingship of Gondor fell on Anárion's son Meneldil. Thus, the Kings of Gondor were descendants of Anárion through many generations until the time of Eärnur.
Dunedain of the South: The descendants of the Exiles of Númenor who settled in the southern lands of Middle-earth, and founded the South-kingdom of Gondor. Their first independent King was Meneldil, the nephew of Isildur, whose line ruled over Gondor for more than two thousand years. Unlike their cousins in the North, the Dúnedain of the South maintained their realm even after the loss of its Kings, and it survived under the rule of the Stewards until the time of the War of the Ring, when the Kingship was restored.
Dunedain: A term used in Middle-earth for the Men of Númenor and (especially) their descendants in Arnor and Gondor after the Downfall in II 3319.
So it seems that Dunedain are the descendants, like WN said, of the refugees from Numenor.
And since I am, of course, of noble blood I guess I can claim either title. So I can still fetch the occasional Elvish Princess. ;^)